Numerous elevated pressure processes utilize solid feedstock, or would benefit from use of a solid feedstock. For example, gasification processes, such as for coal and biomass, may utilize solid feedstock that is fed into a pressurized reaction chamber. For some processes, pressures in excess of 500 psia (e.g., up to 1000 psia) may be utilized, such as for compact gasifiers utilizing processes including an Integrated Gasification Combined Cycle (IGCC).
Initially, the solid feedstock is located in a substantially ambient pressure environment, outside of the system. The solid feedstock must then be moved from the substantially ambient pressure environment to an elevated pressure within the system. To address this pressure change, solid feedstock may conventionally be delivered to a pressurized system by one of a multi-stage lock hopper system, a slurry pumping system, a pneumatic feed system, and a Stamet pump.
Regarding multi-stage lock hopper systems, a low-pressure hopper chamber is first opened only to the outside environment and solids are inserted into the low-pressure hopper chamber at ambient pressure. A valve is then opened and the material in the low-pressure hopper chamber is transferred to a lock hopper chamber by a valve. An inert gas is pumped into the lock hopper chamber, pressurizing the lock hopper chamber. Then, the material in the lock hopper chamber is transferred to a high-pressure hopper before being transferred into the reaction chamber. The lock hopper chamber is then closed, and the lock hopper chamber is bled down to substantially ambient pressure to restart the cycle. As may be observed, this procedure requires compression of the lock hopper chamber, or the loss of reactor gases by back gassing into the lock hopper chamber upon opening. Furthermore, the pressurization of the lock hopper may be a major portion of the cycle time. In view of this, multi-stage lock hoppers suffer from high inert gas consumption, high-energy consumption for compression, and product (e.g., syngas) dilution with the inert gas. Multi-stage lock hopper systems also are only capable of delivering bulk product at intermittent intervals. Intermittent product delivery may be suitable for some gasifiers, such as those utilizing a batch process, but it is not suitable for all types of gasifiers, such as those that require a continuous feed.
Regarding slurry pumping systems for gasification systems, a solid feedstock is suspended in water (or other liquid) to form a pumpable slurry. The slurry is then pumped from the ambient pressure environment, and injected into a pressurized reaction chamber through a high-pressure injector as an atomized spray. Although pumping the slurry requires relatively low power consumption, the water required to create a pumpable slurry is substantially greater than that required in the gasification reaction (i.e., on the order of 65% greater). In view of this, extra energy is required to vaporize and superheat the excess water that does not contribute to the reaction and creation of product. This results in lower overall cycle performance. Additionally, the high velocity spray from the high-pressure injector may cause impact erosion within the reaction chamber.
Pneumatic feed systems, like multi-stage lock hoppers, suffer from high-energy consumption and inert gas consumption.
Finally, existing solid pumping systems, like the Stamet pump, have limited pressure rise and are also vulnerable to mechanical issues, such as erosion.
In view of the foregoing, an improved solid feed system for elevated pressure processes, such as coal and biomass gasification plants, would be desirable.